For the foaming or expansion of molten metallurgical slags, it has been considered necessary heretofore to contact the molten slag with a quantity of water such that the expansion takes place in the proper relationship between the molten slag and the water to obtain complete and uniform expansion of the slag.
To this end, various apparatus has been proposed heretofore. For example, the slag may be expanded in foaming chutes or troughs in which the slag is passed continuously from one end of the chute to the opposite end thereof and is continuously contacted with a water film which is provided on the floor of the chute. This brings about the desired contact of the molten slag with the water to effect at least partial foaming of the slag.
It has also been proposed to foam the slag by contacting the molten slag with water upon rotating bodies which can be formed with plates or vanes and which centrifugally disperse the molten slag as it contacts the water phase.
In yet another system, the slag is subdivided and thereupon brought into contact in the form of small liquid particles, with the water phase. In the latter case, the slag particles are projected through the air, so that they come into contact with both water droplets and steam and thereby both expand and solidify.
Such systems, which have been found to be effective in the foaming and expansion of molten slags delivered at relatively low rates over given time periods from a metallurgical furnace or a ladle, have a significant technological disadvantage in that they are incapable of modification in the foaming parameters, namely, the duration of the prefoaming phase, in accordance with the slag characteristics. In fact, it is scarcely possible with these systems even to control the quantity of water and the duration of contact of the water with the molten slag in accordance with the physical characteristics of the latter.
Since satisfactory and reproducible results require that the slag contact the water for a duration and in an amount which is related to the physical characteristics of the molten slag, it has been necessary heretofore to maintain the physical characteristics of the slag within a narrow range. The physical characteristics of the slag which are most significant in this regard are the temperature and the viscosity.
Difficulties have been encountered, therefore, when the molten slag is at a different temperature or viscosity from that for which the foaming or expansion system was designed.
Technologically, therefore, it was necessary to attempt, by controlling the metallurgical process which generated the slag, to maintain the temperature and viscosity thereof substantially constant.
This could not be achieved with many blast furnace operations, however, in the preparation of Thomas or Bessemer pig iron, because an optimum generation of the primary product, namely, the pig iron, often gave rise to slags with varying physical characteristics.
For instance, it is known that hot slags, as long as they do not contain excessive quantities of lime, are readily foamable. Hematite pig iron slags also have, in general, good foamability or expandability, as do Thomas pig iron slags having temperatures in excess of 1475.degree. C. However, with lower temperatures, the foamability of Thomas pig iron slags declines sharply.
When it is desired to work up slags of poor foamability to so-called blast furnace "pumice", it is found that the foamed or expanded product can contain as much as 60-70% by weight or volume of slag sand, i.e. a predominantly unfoamed product which has less value and desirability than the blast furnace pumice. As a consequence, it is necessary to separate the slag sand from the pumice component.